(1) Field of the invention
The present invention qenerally relates to a pressure control system of a below mentioned "evapo-purge system" for controlling a pressure in a fuel tank of an internal combustion engine wherein evaporated fuel in the fuel tank is treated so that it is discharged into an air intake system.
(2) Description of the Related Art
Evaporated fuel in a fuel tank of an internal combustion engine is treated in a conventional "evapo-purge system" so that the evaporated fuel, generated in the fuel tank and other portions in the engine containing the fuel therein, is to be absorbed in active carbon, the evaporated fuel absorbed in the active carbon then being purged from the active carbon into an air intake system.
The Japanese Laid-open Utility Model Application No. 51-105906 discloses such a conventional "evapo-purge system". A check valve is provided in a path in the conventional evaporated system, which path connects between a fuel tank and a canister. The check valve allows evaporated fuel in the fuel tank to be discharged into the canister if a pressure in the fuel tank is higher than a first predetermined pressure thereof. After stopping the engine, the pressure in the fuel tank is controlled to be not higher than the first predetermined pressure by the check valve.
Generally speaking, a temperature of fuel in a fuel tank of an engine, particularly a temperature of fuel located in a central portion of the fuel tank, increases during several seconds soon after the engine stopping, because heat is given to the fuel tank from a sub-tank. Thus, fuel in the fuel tank continues to evaporate.
The check valve of the above conventional "evapo-purge system" closes soon after the engine stopping, which valve has been opened until the engine stops. The check valve is controlled after the engine is stopped so that a pressure in the fuel tank may be not higher than the first predetermined pressure. Thus, the pressure in the fuel tank increases sharply up to the first predetermined pressure because heat is given to the fuel tank from the sub-tank soon after the engine is stopped as mentioned above.
If somebody opens a fuel-supply-cap of the fuel tank for supply of fuel therein in such condition thereof, evaporated fuel may be leaked into atmosphere because pressure in the fuel tank is higher than the atmospheric pressure (the first predetermined pressure is higher than the atmospheric pressure). This leak of fuel is a fuel economy and/or environmental problem.
To overcome the problem, the Japanese Laid-open Patent Application No. 2-130254 discloses an improved pressure control system of an "evapo-purge system". The pressure control system is used for reducing evaporation of fuel in a fuel tank of an internal combustion engine and reducing an amount of evaporated fuel evaporated in the fuel tank discharged into a canister. A pressure-controlled-pressure-relief-valve and a constraint-pressure-relief-valve are provided in parallel to each other in a path in the pressure control system, the path connecting a fuel tank of an engine with a canister. The pressure-controlled-pressure-relief-valve controls a pressure in the fuel tank to be not higher than a first predetermined pressure, which first predetermined pressure is higher than the atmospheric pressure. The constraint-pressure-relief-valve is controlled by an external command therefor. Both valves are controlled so as to fllow evaporated fuel in the fuel tank to be discharged into the canister or prevent it from being discharged thereinto.
FIG. 1 shows a concept of the pressure control system. The constraint-pressure-relief-valve closes in the engine running state as shown in a step (the term "step" will be omitted for the sake of simplification hereinafter) S1 of FIG. 1 in order to reduce evaporation of fuel in the fuel tank. Thus, the pressure-controlled-pressure-relief-valve controls pressure in the fuel tank as shown in S2 so that the pressure therein may be controlled to be not higher than the first predetermined pressure P1 as shown in S3. The constraint-pressure-relief-valve opens as shown in S5 after stopping the engine until a predetermined period .DELTA. t elapsed as shown in S4 in order to prevent discharging of evaporated fuel through a fuel-supply-cap of the fuel tank at a time of the cap being opened for supply of fuel. As a result of the opening of the valve, evaporated fuel in the fuel tank is discharged into the canister, a pressure in the fuel tank thus decreasing as shown in S6.
The constraint-pressure-relief-valve closes again since the above predetermined period .DELTA.t elapsed as shown in S7 in the engine stopped state. Thus, the pressure-controlled-pressure-relief-valve controls pressure in the fuel tank as shown in S8 so that the pressure may be controlled to be not higher than the first predetermined pressure P1 as shown in S9. Thus, pressure in the fuel tank increases up to the first predetermined pressure P1, which pressure is higher than the atmospheric pressure as mentioned above. Thus, evaporation of fuel in the fuel tank is restrained and discharging of evaporated fuel into the canister from the fuel tank is restrained in the engine stopped state.
The problem of the above mentioned pressure control system is described below. Before stopping the engine, pressure in the fuel tank is controlled to be not higher than the first predetermined pressure P1, which is higher than the atmospheric pressure as mentioned above. Thus, when the constraint-pressure-relief-valve opens since the engine running as mentioned above, pressure in the fuel tank reduces quickly to the atmospheric pressure. Then large amount of evaporation of fuel in the fuel tank may occur as a result of sharp variation of pressure therein.
In particular, a temperature of fuel in the fuel tank is high soon after stopping the engine as mentioned above. As a result of this high temperature of fuel in the fuel tank, an ebullition-under-reduced-pressure-state may occur in the fuel tank. Then if the ebullition-under-reduced-pressure-state occurs, significant evaporation occur and a considerable period may be needed before pressure in the fuel tank reduces to reach a desired pressure so that the evaporation stops.
As a result of the considerable period needed until the evaporation stops, the amount of evaporated fuel discharged into the canister may exceed an absorption capacity of the canister, that is, a saturation state of the canister. This saturation state of the canister may cause evaporated fuel discharged into the canister to be discharged into atmosphere through the canister. The considerable period needed until the evaporation stopping also may interfere with the normal functioning of a fuel-supply-excess-preventing-mechanism so that excess supply of fuel into the fuel tank may occur.